The conventional electromagnetic exploration for deep geo-electrical structure sounding uses a big loop or long grounded wire for the electromagnetic transmitter in order to generate enough transmitting moment. The magnetic field sensors at the receivers are usually air loops of fair size, coils with magnetic flux concentrator, or SQUID (Super-conducting Quantum Interference Device) magnetometers. For the marine electromagnetic exploration, it is easy to tow a long wire or a big loop transmitter. For onshore, the transmitter is usually at a fixed location while the receivers are moving in the areas from about one kilometer to up to few tens of kilometers away from the transmitter. Due to the size and weight of the conventional receivers, it is almost impossible to create a distributed receiver station comprising hundred or thousand receivers.
The one-, two-, or three-dimensional inversions are commonly used for the interpretation. The big transmitter which is a long grounded wire or a big size loop source is known to have source effect, especially when using one-dimensional inversion interpretation. The interpretation results could be different if one changes the actual transmitter location.
There has been a number of different US patents describing the use of electromagnetic deep soundings. The patent (U.S. Pat. No. 4,591,791) of Bostick Jr. X, Francis disclosed an electromagnetic exploration method for geophysical exploration, in which the variations in the earth's magnetic field are measured in two, non-parallel directions at one point in the exploration area. Simultaneously, the variations in the earth's electrical field parallel to the exploration line are measured at a number of points along the exploration line. The patent (U.S. Pat. No. 7,894,989) of Srnka, Leonard J, et al. disclosed a method for determining earth vertical electrical anisotropy from offshore electromagnetic exploration measurements. This method requires both online and offline data, which includes at least one electromagnetic field component sensitive at least predominantly to vertical resistivity and another component sensitive at least predominantly to horizontal resistivity. The patent (U.S. Pat. No. 7,126,338) of MacGregor, Lucy M, et al. disclosed an exploration method using electromagnetic sources aligned end-on and broadside relative to the same or different receivers. The patent (U.S. Pat. No. 4,617,518) of Srnka, Leonard J. disclosed a method and apparatus for offshore electromagnetic sounding utilizing wavelength effects to determine optimum source and detector positions. The patent (U.S. Pat. No. 55,563,513) of Tasci, Tahsin M, et al. disclosed an apparatus and method using an electric generator connected to a transmitter with a length of wire with grounded electrodes. The eddy currents induce magnetic field changes in the subsurface which can be measured at the surface of the earth with a magnetometer or induction coil. The patent (U.S. Pat. No. 7,746,077) of Strack, Kurt M., et al. disclosed a marine electromagnetic exploration method using a towed electric dipole and including a plurality of sensor modules disposed at spaced apart locations along a cable. Each module includes at least one magnetic field sensor and at least one pair of electrodes. The patent (U.S. Pat. No. 7,800,374) of Strack, Kurt M. disclosed a multi-component marine electromagnetic signal acquisition cable and system. The patent (U.S. Pat. No. 7,860,655) of Alumbaugh, David, et al. disclosed a technique for electromagnetically detecting thin resistive bodies in shallow water and terrestrial environments, which includes using a first sensor to perform a first electromagnetic field measurement to obtain first data, which is indicative of the presence of a resistive body. The first data is relatively sensitive to an effect that is caused by an air layer boundary. The technique includes using a second sensor to perform a second electromagnetic field measurement to obtain second data, which is indicative of the presence of the resistive body.
However, none of the above mentioned patents disclose a small size of electromagnetic transmitter with variable spectrum for variable deep sounding penetration and easy movement. Moreover, neither of the above mentioned patents disclose a high dynamic range, high sensitive magnetic field sensor which are easy to be distributed so that a big quantity of receivers could be setup at the field site.
There has been a number of US patents which describe the uses of electromagnetic exploration with seismic. The patent (U.S. Pat. No. 7,328,107) of Kurt M. Strack disclosed a method for interpreting spatial distribution of properties of the Earth's subsurface includes obtaining seismic data over an exploration area of the Earth's subsurface. Controlled source electromagnetic exploration data are obtained over substantially the same exploration area. Further data may include gravity, magnetics, seismic any type and borehole data. Constraints are successively derived from the data sets and also cross checked against reservoir data where available. The patent (U.S. Pat. No. 7,340,348) of Kurt M. Strack disclosed a Method for acquiring and Interpreting Seismoelectric and electroseismic data. The patent application with publication number US20100172205 of Oyvind Hillesund disclosed a method for marine geophysical exploration according to one aspect of the invention includes towing at least one geophysical sensor streamer in a body of water. The streamer includes a plurality of spaced apart electromagnetic field receivers disposed at spaced apart locations along the streamer. The streamer also includes a plurality of seismic sensors disposed at spaced apart locations. The seismic sensors each include at least one pressure responsive receiver and at least one particle motion responsive receiver. At selected times, a seismic energy source is actuated in the water. Particle motion and pressure seismic signals, and electromagnetic field signals are detected at the respective receivers.
However, none of the above mentioned patents disclose usage of the same distributed receivers with magnetic field sensors and geophones or MEMS (Micro Electro-Mechanical System) accelerometers for electromagnetic exploration and seismic exploration. Moreover, neither of the above mentioned patents disclosed a method and apparatus to acquire three-dimensional seismic and three-dimensional electromagnetic data during the same operation, and to derive the magneto-telluric data from the three components of the magnetic field measured by highly populated receivers at the field site.
In order to overcome above shortcomings, inventor had the motive to study and develop the present invention. After hard research and development, the inventor provides a reduced size electromagnetic transmitter so that the transmitter is easy to be moved especially for the onshore exploration. A high dynamic range, high sensitivity and small size magnetic field sensor is also used for easy field setup to make possible about highly populated receiver stations. Besides, a combined field operation of electromagnetic and seismic exploration, and the method to derive the magneto-telluric exploration from highly populated receivers with three component magnetic field sensors are also developed.